Mercedes EGR Valve Test

The EGR valve on this Mercedes MBE 900 diesel engine was under suspicion so we hooked up the software and went into the test mode. The valve can be moved to any desired position using the software which indicates if the EGR is faulty or not. In the video the valve moves correctly when manipulated by us.

It's controlled by an electric motor that I have seen fail on occasion. Usually an EGR valve code will come up once the valve moves in the wrong direction. If you are unfamiliar what this valve does I will explain it's purpose. EGR cooled exhaust was one of the first designs in the engineering department by factory engineers to combat the levels of Nitrogen Oxide or NOx.

By cooling the exhaust before it enters the intake system the NOx is reduced with lower exhaust temperatures and less oxygen levels. The disadvantage of this process is soot levels are increased with cooler exhaust levels in the combustion chamber. The DPF filter takes care of this down the line as long as the engine is worked hard and there are no mechanical issues like extra fuel or oil getting into the combustion chamber.

The onboard computer controls the open and close routine of the EGR Valve. Up to 50% of the exhaust is routed into the engine intake according to the performance setting in the software. Different loads and speeds dictate the EGR settings. When the computer commands the valve to open to a certain percentage and it does not an engine code pops up. Poor engine performance will be the first sign there is a problem.

Without the engine software your hopes are slim to none figuring out the problem. Codes are explained very well and most times it will lead you down the right path. That's it, emissions are here to stay and EGR valves are essential to control NOx.

Mercedes MBE 900 Diesel Particulate Filter Cleaning

The emission controlled diesel engine causes a lot of soot and ash build up through out the intake and exhaust system. The sticky GOO that invades literally every square inch of the engine's components has to be removed by way of passive or parked regenerations or physically cleaning off the build up before it's too late.

The video shows us using a household cleaner to soak the diesel particulate filter from a Mercedes MBE 900 diesel engine. This filter was condemned by one of the local shops we took it to for cleaning. They baked it at extreme temperatures to remove as much residue as possible. In this case the test for flow was too low for them to give it a pass.

So our options were to buy a cleaned DPF from the Mercedes dealer for $800.00 or experiment on our own with 'Mr Clean'. In the past we've used it to flush out cooling systems when there's oil contamination or for removing soot from small emission parts but this was something we wanted to try since there was nothing to lose. Mr Clean is an excellent degreaser so our plan was to let the DPF soak for 2 days and thoroughly steam clean it off.

I'm happy to report that since reinstalling this DPF there has been no fault codes for close to three months. The typical 2631 code (turbo boost performance) has not reared it's ugly head for some time now. Since then we've done 2 more buses with the same success. How long until the DPF starts to get plugged up again is hard to say.

They're supposed to be serviced or replaced around 150,000 km according to the Mercedes estimate but that's up for discussion. The plastic drum we used to soak the filter was a used DEF container. One gallon of Mr. Clean is all it takes to get a good strong solution. The soot and ash collected on the filter is liquified so when you use the steam cleaner it comes out looking really good.

Another thing we do is clean contaminated sensors and reuse them instead of spending 50-60 dollars each. Replacement parts for emission controlled diesel engines are expensive so any shortcuts or experiments that work is a bonus :)

Mercedes MBE 900 Parked Regen

Using Mercedes Software To Perform a Regen from John Whelan on Vimeo.

In 2007 everything changed for diesel engines. The newly integrated emission systems were now the most important cog in the wheel. They had to meet certain parameters according to the onboard computer programming and if they did not an engine code would appear and stare you in the face until the fault was repaired. At first it was frustrating since we had not worked extensively on these emission controlled MBE 900 models.

The Mercedes dealer become regular aquaintances and saw our buses many times over the 5 year warranty period. Engine codes appeared then went away and popped back up again a couple of days later and so on. Once warranty expired we knew it was time to get into these engines more than ever. The online resources called DDSCN was very helpful along with the Detroit Diesel engine software we had loaded up on our laptop.

There was a lot to absorb since everything was monitored by the MCM (motor control module). Turbo boost, EGR and diesel particulate filter cleanliness determined how well these engines were going to run. Once there was a restriction in the DPF a regular code 'dpf out of range high' would pop up until we did a parked regen or repaired a sensor or plugged up sensor air line. Without the diagnostic software we could not do a thing to get these engines back in shape.

The DPF section in the software allows you to watch all of the temperatures in the DPF and DOC. The differential pressure indicated if there was a restriction (3 psi max). The video goes into a parked regen where the temperatures go as high as 1,000 degrees cooking out the DPF. This half hour process does the job and most times it makes a huge difference in driveability.

Another option to troubleshooting was an over the road regeneration where you could monitor the regen process while on a road test and record the data. The Detroit Diesel software is very good for narrowing down a problem. Once there is a code the software can troubleshoot the code step by step which really made life easier for the techs.

The filter cartridge in the DPF is replaceable which we had to do a few times because the regen process was not working. It's similar to an air filter that you clean out and stick back in the cannister. Eventually it's not going to clean up properly over time due to the compounded accumulation of debris. Soot and ash are a never ending problem throughout an emission controlled diesel engine and the MBE had lots of that with sensors plugging up and EGR valves sticking.

They are definitely not the perfect engine when combined with emissions but working on them and gaining experience helped our shop know what steps to take whenever a fault occurred.

Mercedes MBE 900 Diesel Engine Emission Code dpf out of range high

This code we're working on is a "DPF out of range high" on a Mercedes MBE 900 Diesel Engine. What triggers this code is a pressure differential between the inlet and outlet of the diesel particulate filter cartridge. In the video you can see the clamps on the outside of the muffler that can be removed allowing the cartridge to be removed for cleaning.

There are several shops in town that will do a cleaning of these cartridges. It's a two step process depending on how badly the blockage is. When using detroit diesel diagnostic software you can go in and see what range the filter is running at. It's 0 to 4 range with four being severe. Once the engine control module detects blockage it will run a regen while the engine is in service.

 In a perfect world that's all the emission system would need to do running regenerations to keep operating without any trouble. The DPF amber dash light illuminates when the regen process is taking place. There is no worries if it flashes on and off indicating a normal function instigated by the engine on board computer.

Mercedes calls the engine computer an MCM (motor control module). When hooked up with the laptop and software the tech can find the code and click on the "troubleshoot code" button at the bottom of the screen. This directs you to a step by step process to check one thing at a time then moving on to the next step. In this case the outlet braided pressure hose that reads outlet exhaust pressure of the filter cartridge has to be removed and tested for blockage.

It could be blocked with soot and ash build up and the best thing to do is replace it. This fault was cured replacing the hose. If it did not help the filter cartridge may be plugged up so a forced regen is a good idea. Eventually over time the filter cartridge needs to be serviced or replaced. Mercedes recommends servicing at 125,000 km or 75,000 miles.

We have 18 of these diesel engines in our fleet and have a good deal of experience which is your basic "hands on" experience that is of course the best teacher.

TPS Throttle Position Sensor Code

TPS Code Thomas School Bus With an Mercedes MBE 900 Diesel Engine 

The driver called us up on the radio and said he had no throttle. The bus he was driving was 10 minutes out of town. The bus was a Thomas HDX Pusher with a Mercedes MBE 900 diesel engine. The TPS which needs to send a signal to the engine ECU or in Mercedes terms it's called an MCM (Motor Control Module). This was not happening so some diagnostics were in order.

Without a throttle position signal to the MCM shifting and engine power goes crazy. Engine RPM will decrease and the transmission (Allison 3000 series automatic) will not shift properly. The 5 volt signal to the CPC (common powertrain controller) from the throttle position sensor will not happen. The CPC is the interface between the chassis controls and the engine / transmission. In other words it communicates the signals to the MCM.

After finding a proper wiring schematic and checking the circuits we traced the problem down to the TPS itself which as you saw in the video is mounted on the right side of the throttle pedal. It was a funny occurrence because we swapped throttle position sensor units with another bus with out any change. But...the problem was not with the TPS but the wiring to the throttle position sensor connector.

It was hanging by a thread and caused an intermittent throttle signal with a big voltage drop. This causes a bigger challenge for the mechanic doing the troubleshooting. Wiring is tricky at times but the only difference is the time involved to find the fault. So guess what...next time we have a pusher bus with no throttle that connector will be one of the first places to check.

The TPS wiring and switch is on the Thomas Bus (OEM) side so we can't blame the Mercedes diesel powering this piece of equipment. In conclusion it might have been the operator's boot that hit the connector wiring while going into full throttle. It's a hunch but it could very well happen. If you have time leave a comment or a question.

I don't get to questions right away but always check back periodically. Thanks for reading this post and I hope it helped you out. Remember that a diesel engine control module needs to be told what to do. It must have a signal from a sensor before it allows the diesel engine or transmission to operate. It's great technology if you don't get mad at it and lose patience. Work with it and get the experience..... you will not regret the knowledge that you accumulate over time.

Mercedes MBE 900 Diesel Engine Emissions and Engine Codes

EGR - Exhaust Gas Recirculation Maintenance

I want to share some observations and solutions with emissions and engine codes we have been experiencing with the Mercedes MBE 900 diesel engines in our bus fleet. We have a dozen of these engines 2007 - 2008 emission controlled and they are all off warranty so this has allowed us to learn by "doing" with the various emission and engine codes that normally occur. The time we have spent troubleshooting these various problems has taught us a lot and we have adapted to the typical problems that happen with this model engine.

The EGR Cooler above had to be replaced due to a leakage problem. With a coolant leak they will eventually plug up with a sludge build up if it does not get attended to. If the EGR cooler is not leaking and has a soot build up it will require removal and a hot tank cleaning at your local radiator repair shop.

They are not cheap to replace however if they leak you'll experience coolant loss and engine code problems because of the soot wet mess that will develop inside the cooler exhaust core. This will cause poor exhaust gas flow and reduce engine power. The EGR cooler exhaust gases flow to the EGR valve which mixes the cooled gases with the charge air intake reducing NOx (oxides of nitrogen) creating reduced emissions out of the tail pipe.

Charge Air Cooler Failure Mercedes Code 2631

The charge air cooler to the left split open right at the core on the engine side. This problem was hard to detect because of the noise going on when the engine is running.

The reason we found it was because of a pressure check on the cooler itself. There was a 2631 Mercedes engine code which is a "turbo boost performance" issue.

Mercedes has a procedure for troubleshooting this code and it's quite involved. You have to check the boost pressure sensor for soot build up then using the software check the EGR operation along with the intake throttle valve etc. Any part of the engine that isn't allowing full flow including the air intake and boost side will produce the 2631 code.

In this case we have blown charge air cooler causing a big drop in boost pressure. The good part is we found a problem the down side is it's located 10 days away in Memphis. This bus is down until then. NOTE: We used a regulator to pressure test this cooler at 30 psi and made plugs that can be clamped at both ends.

Mercedes Diesel Engine "low doser fuel pressure" Code

The doser code activated because the fuel pressure supply in the system was too low and this caused the regeneration process to fail.

The "doser" system shoots a mist of diesel fuel down stream to cause the regeneration operation to activate at the DPF (diesel particulate filter).

No regen operation meant the DPF was going to build up soot and ash to the point where the engine would eventually shut down. This is serious since the engine control unit is programmed to protect the engine and will not allow it to run when the emission parameters are out of the ball park.

The Mercedes power service literature is very good for troubleshooting information. This test shows how to measure the return fuel going back to the tank. At idle the lines are hooked directly to the secondary fuel filter return and a tee fitting is hooked into the overflow valve.

There are 2 fuel regulators one for the doser fuel side and one for the high pressure injection side (unit pumps). If there is too much fuel returning back to the tank the fuel pressure reading drops.

The fuel pump is a positive flow gear pump that is engine driven. Fuel pressure must get up to at least 65 psi to supply the doser system (does the exhaust regen operation) and the unit pumps which provides the high pressure fuel to the injectors.

Once we changed the 2 valves in the fuel return lines "overflow valve" and "doser fuel pressure regulator valve" the exhaust regen started working again and the engine power improved drastically. So it was a win - win in this case.

If you're interested in viewing more blog posts photos and videos check out mechanicshub.com and look for "blogs". I'm posting under the name "schoolbusmechanic"

Mercedes 900 Diesel Engine Runs poorly after fuel filter change

The Mercedes MBE 900 Diesel Engine. What I have experienced since we acquired these engines in 2005 is they are built well and stand up to the rigorous start and stop routines involved in a school bus fleet. The pre 2007 engines are not emission controlled like they are today and have given us very little grief. However I would like to share a tip on a troublesome problem we had with a 2005 freightliner conventional school bus.




The Video I made above explains how to properly bleed these engines. Strangely enough we were told to crank over the engine continuously after a fuel filter service until the engine fuel pressure built up. At first we were scratching our heads thinking we're living in the new millenium and this diesel engine has no priming pump and only one starter...that was destined for failure eventually from the constant load it would have to endure turning the engine over and over just to prime the fuel filters.

We tried to innovate and pressurize the fuel tank and crack a line at the secondary fuel filter to help the system prime up faster. Unfortunately some of our new buses had fuel guage sender problems and there were buses running out of fuel out on the road. There was no choice but to load the hell out of the starter to get it going again.

By the way if you loosen the fuel line from the unit pump to the head the line must be replaced. Recommended by the dealer / engineers. We were forced to crack the lines only once and re-tightened them after start up and did not have any leaks. So I imagine the odd time the fuel line will not seal properly after it has been disturbed?  

Engine Runs Poorly After Changing Fuel Filters

We recently experienced a problem with one of our MBE 900 diesel engines after a fuel filter change. It ran poorly and was not picking up fuel. It eventually kicked in and ran fine for a few days. However it died on it's bus run and had to be towed. It ran like it was starving for fuel like air in the system. The engine would idle fine but crap out under a load.

One thing noticeable was when priming the fuel system with the pump kit the pump just kept filling the lines and filter without the typical solid feeling at the pump handle indicating the system was primed and ready to go. Eventually the Mercedes diagnostics instructed us to check the return fuel volume from the head and the unit pumps.

There should only be 1.7 Liters of fuel returning to the tank every minute at an idle. We had double that...so it ended up being the overflow valve (similar to a check valve) which is in the return fuel plumbing to maintain around 60 psi. It was not doing this and caused a loss of back pressure in the return system which caused the hard starting, rough running condition. Now our shop will recognize the problem after these symptoms pop up again.

Mechanic Tips-School Bus Fleet Preventive Maintenance

Every Fleet Mechanic will have there own ideas on how to set up the best PM program for their equipment which will vary for different locations. With different climates, road conditions and power trains the procedures will never be exactly the same. Our shop likes to condense the annual PM since the regular checks are carried out every 5000 Kilometers (3000 Miles). Oil changes are done every 15,000 Km with an oil sample to be analyzed. Fuel filters are replaced at every oil change.

The Allison 2000 and 3000 Transmissions  are all running on TES-295 Synthetic oil which has drastically cut down on repairs. Right now we change the oil every 3 years, the newer buses have prognostics which tells us with a dash light indicator when the filter and oil needs to be changed. Air filters are replaced every year regardless of the mileage. Steering Filters are serviced along with changing the oil every 2 years. Valve Sets are carried out according to the Manufacturer i.e Cummins every 5000 Hours or 240,000 Km (Electronic Models). With the introduction to emission controls in 2007 we now have to perform manual regens if required and replace the DPF (diesel particulate filter) every 150,000 Km (90,000 Miles)

Air Dryers every 2 years with a new unloader kit and dessicant filter. Dirt is a factor on school buses so we service interior heater filters and the entire lighting system (Non-LED lights). We check the starting and charging systems including the battery pack. If necessary we will run tests on the batteries and alternator. years ago alternators and starters were removed for rebuild but the time and parts did not add up as an economic strategy so we decided to give these components a 5 year life span before determining if they would need replacement. The cost has come down so much on electrical reman units, it's feasible to go this route with the added bonus of a warranty period.
 
Spring brake chambers are replaced every 5 years regardless of mileage and condition. The Mercedes MBE 900 requires a coolant and thermostat change every 2 years, the other diesel engines in our fleet do not have a time or mileage limit but we're working on that to prevent antifreeze deterioration and thermostat failures. Our tires tread depth is measured every Summer and pressures checked during each service.

Common sense plays a role with any preventive maintenance program and we found the best way to set one up is to sit all the Mechanics down in one room and go through the fleet  figuring out the best program to cover all the bases. Breakdowns will quickly eat up your labor time and parts budget like wildfire so keeping on top of maintenance with a feasible program will pay you back 10 times over.                                                                     

Bleeding A Diesel Fuel System - Mercedes MBE 900 Diesel Engine

Here's a short video on bleeding the fuel lines on a Mercedes MBE 900 diesel engine. I'm using a tool made by Kent Moore with quick couplers that makes this a simple process. In 2006 we received 6 of these engines that didn't come with these couplers and at the same time the fuel sender wasn't accurate. There is a technical service bulletin that explains how to test the fuel tank sender from the OEM (Thomas Bus) or the operator will notice the fuel gauge is off track.

It was no surprise when we had a few incidents of buses running out of fuel. Back then all we could do was crank over the engine and crack one of the fuel lines or lightly pressurize the fuel tank to get the engine fuel system bled properly. Eventually we ordered the bleeding coupler kits and improvised a steel tank and hose slightly pressurizing it with shop air which worked great, but a little crude.

The Kent Moore J-47912 tool was the answer, sometimes it pays to give the fuel system a quick bleed after replacing fuel filters during a service.

Bleeding Diesel Fuel System - Mercedes MBE 900 Engine

We learned the hard way & experienced troubles trying to prime the Mercedes MBE 900 Diesel Engine. It was a chore trying to get it started again after it ran out of fuel.
Pressurizing the fuel tank and bleeding the system at the fuel lines was tedious and a crude method to say the least.
Most of these engines come from the factory with a quick coupler located at the fuel filter housing. The 2005 and earlier models have to be installed with the new coupler, a kit can be obtained from the Mercedes dealer.


We acquired the female quick coupler from the local dealer and added a  hose along with a stainless steel pressure tank similar to a lawn fertilizer set up.  There isn't a lot of  pressure required to bleed the fuel system, all it takes is 5 p.s.i or better. There is a bleeding apparatus available that we are looking at picking up. I will re-post when it arrives and give it a test drive.




This tank has to be topped up with diesel and pumped continuously while a second person cranks over the engine. It works great and saves a lot of wear and tear on the starter!

Note: there is a Technical Service Bulletin on the Thomas C2 School Bus models having a defective fuel gauge senderThe sender must be removed and tested with an ohm meter. We had some incidents on one of our C2 units running out of fuel prematurely.

Once you get this pressure tool all set up it will make removing the air from a Mercedes MBE 900 Engine fuel system much easier.

Mercedes MBE 900 Diesel Engine - Changing Thermostats

MBE 900 Diesel Engine Specs:

Configuration: inline 6

Displacement: 7.2 Liters
(439 cu in)

Bore: 4.17 in (106 mm)

Stroke: 5.35 in (136 mm)

Weight: 1,362 lbs (618 kg)

Electronics: DDEC® VI

Compression Ratio: 18:1

The local Mercedes dealer recommended to replace thermostats and coolant every two years on the MBE 900. Coolant breaks down over time and becomes acidic chowing down on gaskets and engine parts. On a conventional and pusher bus this is an easy operation. Three bolts and upper rad hose is all it takes to get to the thermostats.

The two thermostats come with a split o-ring that fits around the edge of the stat and nestles nicely into the bore of the thermostat housing. Below is a short video of a Mercedes MBE 900 diesel engine idling in a pusher bus. These engines come with  EGR and DPF (Diesel Particulate Filter) systems to control emissions. Check out how the emissions are controlled using these components Right Here.

Diesel Engine Cranks Does Not Start

If a diesel engine cranks but does not start the first thing you want to look at is the power feed to the engine controls. In this case it's a Mercedes mbe 900 diesel engine in a 2007 conventional school bus. The first check is the fuse block since this powers up the ignition relays that operate the engine control modules.

Here is the Engine Power Distribution Module (photo below) with the relay that sends a signal to the engine control module.
The wavy line on circuit here has a ground to one side. With no ground the coil does not energize which will not allow the circuit to close.



Mercedes diesel engines have 2 modules, one on the engine and the other inside the cab. The ignition switch activates the relays that gives us the start operation. We found voltage to the cab module but when the engine was cranked over there was a voltage drop down to '0' Volts.

Tracing the wires and checking for continuity was the first step since a voltage drop means a resistance problem in one of the circuits. With the help of the Thomas Bus Tech Line we found the circuits to both relays then found the ground to be non-existent for both ignition relays.
These 2 ground wires (photo below) energize the coils within the relays. No grounds of course caused these relays to be become inactive.
After running a temporary ground the engine started. So obviously we had to check the ground connections on the firewall and the frame. After some digging there was a broken connector to a frame ground below the cab. Once repaired, the engine was back to normal once again.
The photo below shows the broken ground cable found just below the cab firewall on the outside of the frame rail.


This is Mechanical information that points out that a bad ground can cause a lot of electrical problems and should always be checked first starting at the batteries. A digital volt meter will tell you a lot with voltage and resistance readings. The higher the resistance the lower the amperage.

Mercedes MBE Diesel Engine Air Compressor Cycles Too Often

We recently had a problem with a C2 School Bus that had a air system problem. The compressor was unloading every minute, discharging the air dryer. The compressor and air dryer were both doing their job. The air system build time was OK and the air dryer [fyi: filters the air and gets rid of moisture /oil] was doing it's part.

So we focused on an air leak in the unloader circuit. Getting back to basics the air compressor is controlled by the governor which cycles the compressor between 90 psi and 125 psi. This extends compressor life allowing it to cool and take the load off until the air system pressure drops back down to 90 psi.

Also to remove the moisture and oil residue the air dryer unloads at 125 psi as well spitting out the contaminants.
So eventually we found a leak at the unloading air line to the compressor head. The engine is a Mercedes MBE 900 and the compressor is made by Mercedes. The main problem was the fitting which is a push and lock style that leaked down fast enough to allow the unloading air pressure to drain off to atmosphere and cause the compressor to kick in and go through it's cycle once again. The advantage of having an air dryer is the fact that the abnormal discharge of air every minute is very obvious.
So once the part [Thomas # 001F/23-13301-412] was replaced everybody was happy again. So if you have a constant discharge of the air dryer way before it's time it's most likely an air leak in the unloader circuit for the compressor. I hope this mechanic information has helped you...